High Speed serial interfaces straddle many standardisation efforts; these include the DigRF/M-PHY interface developed by the MIPI (Mobile Industry Processor Interface) Alliance, Universal Serial Bus (USB) interface, serial Rapid Input Output (sRIO) bus interface, Common Public Radio Interface (CPRI), Open Base Station Architecture Initiative (OBSAI) interfaces, etc. Key requirements for such high speed interfaces include: high baud rates with minimal power consumption, low bit error rate and minimal EMI (Electromagnetic Interference) emissions. Many electronic products that include high-speed serial interfaces also contain wireless functionality. A mobile telephone device is one such example of an electronic product that includes high-speed serial interfaces.
The MIPI developed M-PHY interface configured in DigRF mode is a high speed interface comprising requirements for EMI that are particularly acute due to the fact that the interface is connecting the Radio Frequency (RF) integrated circuit (RFIC) transceiver device and the baseband integrated circuit (BBIC) processor in a communication device, and utilises data rates corresponding to the frequencies of cellular bands on radio platforms. The proliferation of embedded clock schemes, such as 8b/10b encoding in serial interfaces to facilitate clock data recovery, has an undesired consequence of shaping the resultant EMI spectrum.
Repetitive patterns in data transmissions cause EMI power to be concentrated in discrete frequencies. Such frequency spur generation in the EMI profile of an interface can impair cellular reception, if it occurs at frequencies corresponding to those of cellular bands on radio platforms. In the MIPI M-PHY/DigRF standard, an encoded frame consists of a synchronisation pattern, a start of frame section, a header section, a payload section and an end of frame section. Accordingly, consecutive frames comprise a significant amount of repetition. Furthermore, due to the 8b/10b encoding scheme employed, all the symbols are transmitted on a 10 UI (Unit Interval) boundary. Since most interfaces tend to be synchronous machines, there will typically be a repetitive fixed integer SI (Symbol Interval) boundary between successive frames, exacerbating the problem of repetitiveness.